T169 Proterozoic Crustal Evolution of Southern Laurentia: Tectonic Settings, Petrology, Geochemistry, and Geochronology I

Tuesday, 2 November 2010
8:00 AM - 12:00 PM
Room 302

REFINING THE EARLY HISTORY OF THE MOJAVE-YAVAPAI COLLISION: PRE-COLLISION RIFTING AND POST-COLLISION GRAVITATIONAL COLLAPSE

Paper number: T169-1
Program: Technical Sessions
Time: 8:00 AM – 8:20 AM

Authors

Abstract

Previous studies in the Paleoproterozoic orogen of N and NW Arizona outlined a history of protracted accretion and continental growth at the southern margin of Laurentia ca. 1.74-1.68 Ga, and provide a framework for more focused recent studies that probe the details of its earliest deformational history. These recent studies reveal evidence for 1) back-arc rifting prior to, and 2) gravitational collapse immediately following, initial collision of the continental Mojave block with Yavapai arcs during the regional D1 event (1.74-1.71 Ga).

Mafic and ultramafic rocks are distributed widely throughout the isotopically mixed crust of the Mojave-Yavapai boundary zone as well as the isotopically evolved Mojave crust. In the Hualapai, Cerbat, and Peacock Mountains and the Cottonwood Cliffs, NW AZ, ultramafic and mafic rocks (including meta-pillow basalts) occur within bimodal sequences interlayered with compositionally immature siliciclastic metasedimentary rocks and locally, metacherts and calc-silicate rocks. We interpret this assemblage to have formed during rifting of Mojave continental crust, perhaps in a back-arc setting prior to D1, a process that can also explain the mixed isotopic signature of the boundary zone.

Regional decompression following D1 has generally been attributed to erosion-driven exhumation; however, structures in migmatites in the southern Hualapai Mts. (SHM), AZ, provide evidence for at least local gravitational collapse. Similar structures recognized elsewhere in NW AZ suggest that collapse following D1 was likely regional in scale. Structures within migmatites in the SHM attributed to regional D1 deformation comprise a foliation (S0/S1) that is axial planar to mesoscopic recumbent folds (F1) and a gently E-W plunging lineation (L1). N-striking, conjugate, extensional shear bands offset, sole into, and reactivate the S0/S1 foliation and record subhorizontal, coaxial, E-W extension. The highest P-T determinations are ca. 800°C and 0.9 GPa; the lowest, representing part of the decompression path, yield 660-670°C and 0.41-0.48 GPa, indicating >0.4 GPa of isothermal decompression. We suggest that conditions in the SHM preserved pre- and post-D1 extensional fabrics that have been overprinted by D2 deformation elsewhere in the SW US.